The subject matter of this disclosure is generally related to data storage systems that are used to maintain relatively large data sets and support a relatively large number of concurrent users. Some aspects are more particularly related to a data storage system that uses automated tiering to exploit the service efficiencies of multiple classes of storage. The basic building blocks of a data storage system may include computing nodes that provide access to data storage devices such as disk drives and flash drives. The data storage devices can be organized into hierarchical tiers. For example, each tier may include storage devices of a particular technology type. In general, different technology types correspond to different performance characteristics. Thus, tiered storage provides multiple storage performance classes. However, federating external storage resources with local storage resources in a data storage system that uses automated tiering may present some problems.